QUANTUM-CHEMICAL INVESTIGATIONS OF SMALL MOLECULAR ANIONS

Recent large-scale ab initio calculations for small negative molecular ions are reviewed. Accurate equilibrium geometries are established fo r several species like NH2-, HCC-, NO2-, CH2N-, C-5(-) and C-6(-). Pre dictions are made for various spectroscopic properties like vibrationa l frequencies, rotational constants and infrared intensities. The effe cts of a shallow energy minimum in the T-shaped configuration on the r ovibrational term energies of HCC- are investigated. The calculated vi brational structures of the photoelectron spectra of SiH3- and CH2N- a re in very good agreement with the experiment. The present calculation s support the assignment of an absorption observed at 608nm in a neon matrix to the ($) over tilde (CIIg)-I-2<--($) over tilde X(2)II(u) tra nsition of C-6(-). Electron affinities are obtained with an accuracy o f 0.05eV or better. Spectroscopic properties of intermediates in simpl e S(N)2 reactions are calculated.